Display Types and Technologies
Which type of display is best for you?
Selecting the best display for a home theater today presents both wonderful opportunities as well as challenges and confusion. While there has never been a such a varied selection of high performance, reasonably priced displays, it can be difficult to navigate among the technologies to find what is best for a particular application. The fact that performance continues to improve while prices continue to fall only complicates the situation. In this section of WalVisions we hope to provide an overview of the options available, and to thus help guide you in your selection of the best display type and technology for your requirements.
Our discussions here will go into the important characteristics of the display types. For your particular home theater, you must consider other factors as well, such as how big an image is desired, the necessary display resolution, and the brightness and contrast requirements. Answering those questions along with the information presented here should take you a long way in your selection process. We won't go into specific manufacturers and models, which frequently change, but checking and comparing the latest specifications should give reasonable guidance as to projector performance. Always be cautious of listed specifications,
however, as various manufacturers may measure differently, and some marketing personnel may take liberties in listing specifications. It's always best to
supplement manufacturer's specifications by reading independent reviews and getting a
personal demonstration, if possible.
There are two fundamentally different display formats for home theater - self contained displays and front projection displays. The self contained displays are those boxes that are generally referred to as televisions, monitors,
or rear projection units, and they emit light while absorbing much of the room light. On the other hand, the front projection displays consist of a projector and a separate screen, and generally make a much larger image. Since there is a larger image area for the projector to illuminate and since the room light will also be reflected by the screen, a front projection system
almost always requires a room with controlled lighting.
Self Contained Displays
CRTs (cathode ray tubes) have been the standard for "television sets" for many years, but
have now been almost completely displaced by newer flat screen technologies. They are the bulkiest and are least in fashion today, but can make the best images. The CRT strengths are great black levels, good colors, good brightness, natural looking images, good reliability and relatively low costs, while the disadvantages are limited maximum image sizes, bulk, and the “uncool” factor.
LCD Flat Screens
The LCD flat screens are a technology that is continually improving, and has gotten a big boost from its current widespread use in computer monitors and laptops,
not to mention the enormous capital investments that have been
made in manufacturing facilities. Probably the biggest advantage of LCDs is the slim form factor - they can be mounted on stylish stands or simply hung on the wall in a non-intrusive fashion. They generally have excellent resolution and adequate brightness, colors and black levels. The disadvantages can be limited viewing angles, higher black levels that "wash out" the image,
not-as-good color fidelity, some blurring with motion, and higher cost, particularly in larger sizes - but these factors are improving, so check before you buy. The smaller screen sizes
now dominate the competition with CRTs, while the larger sizes compete with the plasma displays.
LCD's generally make good choices for brighter rooms.
Newer LCD panels
are being developed that use LEDs and the back light
source, rather than CCFLs (florescent lamps).
Depending upon implementation, the LEDs can provide
better color, make for thinner displays, and can be
globally or locally dimmed to provide outstanding black
levels and contrast. This is an area to watch if
you're interested in the best performance possible, but
expect pretty high prices in the early stages of this
The plasma category has been growing and improving for a number of years, and now the technology is capable of excellent images. The plasma technology shares many general characteristics with the LCD types, but are a little more bulky, come in larger sizes,
and don’t have viewing angle restrictions. The
plasma displays also generally make better overall pictures,
exhibiting better color and black levels. On the downside, historically plasmas have had a tendency to take a image "burn" - a fixed image left on for an extended period may
become imprinted on the screen, similar to monitors in airports that show arrival/take off times. This generally shouldn't be an issue in home theater applications, however. Plasma lifetimes have also been an issue, but now manufacturers estimate
up to 60,000 hours of life for some models. Plasmas also
have a glass front that can be "mirror-like" (reflecting room
objects) in brighter environments, unlike LCDs that have a
non-reflective outer plastic layer. Plasmas generally make
better choices for darker rooms.
(Organic Light Emitting Diode) displays are in their
infancy, and are not available in home theater sizes;
They are only available in small sizes and at high cost
per unit area. But they show great promise as each
pixel is its own source of light, and the pixels can be
turned completely off to create pure black. Thus
they can have incredible contrast, great color, and
relatively low power draw (they only make light when
light is needed). This technology could wind up as
the king of the hill in home theater flat screen, but
must overcome the large head start and investment into
LCD and plasma flat panels.
Rear Projection Displays
Rear projection displays basically consist of an enclosure containing a projection system in which the screen is illuminated (projected onto) from the inside of the enclosure, but viewed from the outside. Usually there will be a mirror, and sometimes two, to "fold" the projection path to reduce the cabinet size, but even then rear projectors generally are fairly deep, particularly compared to plasma and LCD displays. Other than the larger system size, the most notable characteristics of rear projection systems are the screen characteristics and the projection technology.
Virtually all rear projectors use screens that are designed to direct most of the light to the viewing audience, and this causes the image brightness to vary depending upon the viewing angle. This is particularly pronounced in the vertical direction (the image may dim significantly as the viewer stands up), but is also present in the horizontal direction. Most casual viewers don't appear to notice the viewing angle restrictions, but you should check for yourself before buying.
The actual projector utilized in a rear projector can be one of a number of technologies, including Cathode Ray Tube (CRT), Digital Light Processing (DLP), Liquid Crystal Display (LCD), and Liquid Crystal on Silicon (LCoS). For information on these particular projection technologies, see the below discussions under Front Projection Displays.
Note that rear projection displays are losing ground in the
marketplace due to the increased competition from LCD and plasma
displays in larger sizes - the larger enclosure size coupled
with the viewing screen characteristics are making the rear
projection display category a hard sell.
Front Projection Displays
Front projection is in a clearly different category than the self-contained displays. A separate video projector projects the image onto a detached screen some distance away. The greater the distance, the larger the image - and in all practical home theater applications the maximum size is only limited by the throw distance (the ability to get the projector far enough away) and image brightness (the larger the image, the less the brightness per unit area). Thus, unlike the self -contained system where you get the size you buy, with front projection you can determine the size you want and then get an appropriate screen and install the projector
appropriately for that size. If desired, you can even
remove and reinstall the projector later to achieve a different size. This is a big advantage of front projection - large image sizes, and the projector size is relatively small, and is the same regardless of image size.
The biggest disadvantage of front projection, other than additional initial setup of the projector/screen combination, is the requirement for a controlled light environment. Screens work by reflecting the projected light back to the viewing audience. Any stray light in the area can also illuminate the screen, and this light will provide an "always there" background illumination that will tend to wash out the image. Additionally, projectors have a fixed light output capability. As the image is made larger, this fixed amount of light has to be spread over a larger area, so the brightness per unit area is lowered. Thus the larger the image, the greater the need for light control. This is discussed further in the
Light Levels, Contrast, Gamma section.
Front projector are available in a number of technologies, and are the same types as available in the rear projector category – CRT, LCD, DLP and LCoS. These technologies can all be excellent, and each has its strengths and weaknesses. Here’s a brief rundown for each of these projection technologies.
CRT projectors are the earliest type of mass produced video projectors. They have three CRTs ("tubes"), a red, a green, and a blue, each with a projection lens to project each colored image onto the viewing screen. The three colors are carefully aligned to overlap and make a seamless, full color picture. CRT projectors continually improved from the 1970s through the 1990s, at which point the technology peaked with very high resolution display of computer graphics (about 2500 by 2000) for commercial applications. These projectors were very expensive, had a limited market, there was little room to make significant advances or cost reductions, and other technologies (LCD, DLP) were making their introductions. Therefore development largely stopped, but the technology had advanced to a high level, even beyond the highest HDTV resolution of 1080p. While difficult to find as "new", refurbished CRT projectors are available, and many of these projectors are still in use in home theaters. Many experts and owners still think that the CRT makes the most natural, film-like image, and clearly has the best black levels of all the technologies.
A big advantage of the CRT is that light is generated as it is needed, so if the image is totally black, no light is created and the image will be totally black. The other technologies all have lamps that are always on, and the light is attenuated by the display device on a pixel by pixel basis to create the image. When black is required, the devices go into full attenuation, but invariably some light leaks through so a full black image has a background “washout” level. CRTs also naturally have perfect "gamma", which refers to how various brightness levels appear on the display, as the television system was designed for use with CRT displays. Additionally CRTs are considered to be "variable resolution" displays, as they can react to the incoming signal format and adjust the spacing of the rows and columns to exactly display the source resolution. This enables viewing sources in their native resolution, and also enables using a scaler to select the optimum resolution for the source material, viewing distance and projector model.
The disadvantages of the CRT starts with their size and setup complexity - they are easily the biggest projector type, and are the most difficult to install, set up, and maintain. This is why almost all dealers have stopped selling them. They also have light output limitations, and at the highest "white" levels can start to "bloom" (lose focus). CRTs may be most appropriate today for the home theater owner who appreciates the technology, is technically inclined and can maintain the projector himself.
LCD projection technology is similar to that in the LCD flat panels displays, but the
actual LCD panels used inside the projector are much smaller, typically being less than 1 inch diagonal. These LCD panels consists of small liquid crystal cells laid out in rows and columns, with each cell being a "pixel" (picture element). Polarized light is passed through each pixel, and the amount of light which passes is controlled by a voltage applied to the cell. Most LCD projectors have three panels - one is illuminated with red light, one with green light, and one with blue light. The light that passes through the panels is optically combined and sent through the projection lens to the viewing screen. Since the pixels are well defined, if the resolution matches the output resolution from a computer graphics card,
a nearly perfect pixel by pixel display will result. This computer display capability fairly quickly got the LCD projector into commercial markets, and it became the first technology to start taking market share from CRT projectors.
The early LCD projectors were characterized by poor black levels and poor colors; they had no place in a home theater. Furthermore, the pixels could readily be seen at normal viewing distances, appearing as a mesh of horizontal and vertical lines now know as the "screen door effect", which is a result of light blocking control electronics which must be placed beside each pixel. The latest
home theater models, however, now have true HDTV resolution, reasonably good black levels and very good colors.
Some projectors are also now equipped with dynamic iris systems
which can reduce the illumination for darker scenes; this is the
rough equivalent to reducing the lamp illumination, which in
turn reduces the image washout that is most apparent in dark
scenes. The weaker points for LCD projectors are slightly slow response, some
unevenness in illumination, and, while much improved, the
elevated black levels and the "screen door effect" from closer viewing distances can still be issues for the critical eye. Advantages are ease of setup and relatively low cost - you can get a very big and very good picture with little effort. These models that have been created for home theater represent a real option for many home theater applications.
The DLP technology is a direct competitor to LCD projection, and arrived in the marketplace shortly after the first LCD projectors. The heart of a DLP projector is the DMD (digital micromirror device), that at less than one inch diagonal is made up of a series of rows and columns of tiny mirrors, each being a "pixel". Each mirror can be at either of two positions - in the ON position it sends incident light from the projection lamp out through the projection lens to
that pixel's location on the viewing screen, and in the OFF position it sends the light to a black absorbing panel. The mirrors can switch positions thousands of times per second, and the ratio of time spent in the ON position to time spent in the OFF position determines the pixel brightness as seen by the eye.
Color is achieved in one of two ways. The "three chip" projectors are like LCD projectors in that there are three DMDs, one is illuminated with red light, one with green light, and one with blue light. The DMDs each determine the amount light that passes from each pixel, and the passed light is optically combined and sent via the projection lens to the viewing screen where the three colors combine to create a full color image. The other method is the more common single chip, color wheel method. In this case the light illuminating the single DMD is first run through a color filter
wheel (cycling among red, green and blue), and the DMD is synchronized to simultaneously display the
image corresponding to that color. Thus the display on the screen
instantaneously flashes red, blue and green images in succession, and the eye, being too slow to see the individual colors, integrates the three colors to "see" a full color image. As you might expect, the single chip DLP projectors are less bright and less expensive than the three chip models, although they may actually have higher contrast due to the simpler optical system.
The advantages of DLP are very uniform illumination with little "shading" (variation in color or brightness across the screen), very good (but not perfect) black levels, a high pixel "fill factor" (the gap between pixels is very small, so there is little "screen door effect"), and they can be easy to setup and operate.
One of the disadvantages of DLP is that the gray scale creation process is "linear", rather than the "geometric" way the eye processes light. We can see over a very large range of luminance levels, and we see every doubling of light level as roughly equal steps. But the smallest pure DLP brightness is perhaps 1/256 of the whitest
level, and this is "only" 9 doubling steps (256-128-64-32-16-8-4-2-1) while the eye can see many times that amount. See the
Light Levels, Contrast, Gamma section. To generate the lowest light levels the DLP has to "dither" - it uses a small block of pixels making some very dim and others black to create an average level for that area that is lower than what any single pixel could produce.
For the single chip DLP projectors, the biggest disadvantage is the "rainbow effect"
(but see the next paragraph!), which is a byproduct of the image being produced with only one color present at any instant. Under certain image conditions and with eye/head motion the eye can separate the colors and the rainbow effect is seen.
A special pattern, the Rainbow Finder
test pattern (WV-14), has been created here at
WalVisions to help in evaluation the rainbow effect. Although many people cannot see this, and the effect has been significantly improved, there are some views who find it troublesome.
One of the more recent
projection related developments is the LED projection lamp,
which has been developed to replace the conventional arc lamps
used in projectors. While the LED lamp isn't quite as
bright yet, costs more and may have some color stability issues,
it has some characteristics that have the potential to
significantly improve the quality of signal chip DLP projectors
in particular. The LED lamp system is made up of red,
green and blue LED modules, and the illuminating colors can be
electronically switched ON/OFF very quickly and independently,
so the color wheel is no longer be necessary. The fast
color switching time means the rainbow effect can be reduced,
possibly even virtually eliminated. The colors can be very
pure, without using filters that can waste some light. The
lamp illumination levels can be dynamically changed (lowered) to
really help in the display of dim images by reducing the need
for dithering. And finally, the lamp life should be
significantly longer than the life of the conventional arc
LCoS projectors use liquid crystals to control light, but in a different way than the conventional LCD projector, in which light passes in one side of the LC layer and out the other side. The LCoS device starts with pixel control electronics on silicon (an integrated circuit), then adds a "mirror" layer, and then places a liquid crystal (LC) pixel layer above the mirror. The light to be controlled enters and passes through the LC layer, reflects off the mirror and returns again passing through the LC layer. The control electronics below each LC pixel effectively controls the pixel brightness by determining how much light gets through the LC layer and out the optical system to the screen.
The biggest advantages for LCoS are that there is almost no "screen door effect", and the pixels can be placed very close together for high resolution in a small device (well less than 1 inch diagonal) and thus LCoS can have a potentially relatively low cost for very high resolution. The disadvantages have been in achieving good contrast ratios and brightness/color uniformity across the screen, but good progress has been made recently on these fronts.
The latest LCoS projectors from both JVC and Sony provide full
field contrast ratios that compete with CRT projectors.
All Lamp Based Projectors
Lamp based projectors, such as LCD, DLP and LCoS, all will share some additional characteristics that should be noted. When zoom lenses are included, usually the full specified brightness will be achieved only with the zoom set for the largest image
size. Ideally the electronic keystone correction should not be used, as it will introduce artifacts and reduce resolution. If non-standard image "offsets" are necessary for your application, try to get a projector that can use lens shift to position the image correctly on the screen, rather than having to angle the projector. All these projectors will require periodic lamp replacements. You should be aware that the typical lamp life is about 2000 hours, that the replacement costs generally run several hundred dollars, that lamps can loose brightness and change colors as they age, and that lamps can fail prematurely. We recommend that you consider lamp comparisons in the evaluation process for finding the best projector for your application.
Also note that virtually all projectors have fans for cooling, and these fans make noise and usually exhaust warm air. If the projector is going to be near the viewing area, check to make sure that both the noise and the exiting warmed air will not be distracting for the viewers.
Also check that there is no stray light exiting the projector
(through the fan or ventilation openings) that will distracting
for the viewers.
While you might get the idea that none of these systems is perfect, generally speaking all these projection technologies are at least adequate, and, with the exception of CRTs, are continually improving even as the prices drop. Personal preference will play a big role is which is best for you – take your time in evaluating the various options to determine what type is best for you. In addition to performance issues, you should consider ease of operation and setup, cosmetics, power requirements, and maintenance costs. We hope that this web site’s
Display Performance and
Test Patterns sections will be helpful in your education and evaluation.